1. Field of the Invention
The present invention relates generally to an anti-skid brake control system for an automotive vehicle. More specifically, the invention relates to an anti-skid brake control system associated with a vehicular speed data projecting means which exhibits improved projected vehicle speed derivation characteristics, particularly with respect to variations in road friction.
2. Description of the Background Art
It is well known that vehicular braking performance becomes optimum when wheel slippage becomes a certain value, e.g. 10% to 20%. Therefore, as is well known, anti-skid or anti-lock brake control is generally performed for maintaining the wheel slippage within a predetermined optimal range. For this purpose, wheel slippage is monitored for controlling operational modes of a vehicular braking system for increasing braking pressure in an APPLICATION mode, holding braking pressure constant in a HOLD mode and decreasing braking pressure in a RELEASE mode. In the vehicular anti-skid brake control, the APPLICATION mode is selected in a normal operational condition for allowing linear deceleration of the vehicle with increasing braking pressure. Anti-skid control is initiated upon detection of wheel deceleration during braking operation, in a greater magnitude than a preset deceleration threshold to switch operational mode of the brake system from APPLICATION mode to HOLD mode. Therefore, at the initial stage of an anti-skid brake control cycle, the braking pressure is held constant in the HOLD mode at an increased pressure, when the wheel deceleration exceeds the wheel deceleration threshold. Because of the increased pressure in this HOLD mode, wheel speed further decelerates at a greater rate than deceleration of the vehicle speed. Therefore, wheel slippage which represents a ratio of difference of the vehicle speed and the wheel speed versus the vehicle speed, becomes greater than a target speed which represents the optimal wheel speed for obtaining optimum vehicular braking characteristics. When the wheel slippage becomes greater than a wheel slippage threshold and thus the wheel speed decreases across the target speed, operational mode of the brake system is again switched into the RELEASE mode for decreasing the braking pressure for resuming the wheel speed toward the target speed. Hence, the wheel speed passes the target speed, and wheel acceleration increases across a preset acceleration threshold. Then, the mode is again switched into the HOLD mode. Because of decreased braking pressure, the wheel speed overshoots the vehicle speed and then decelerates to the vehicle speed. Accordingly, the wheel acceleration again decreases across the acceleration threshold, and the mode is switched to APPLICATION mode again.
In the anti-skid control set forth above, it becomes necessary to monitor the vehicle speed for deriving the wheel slippage. It is possible to directly measure the vehicle speed by means of an appropriate sensor, such as a doppler sensor. However, such a sensor is unacceptably expensive and therefore is not practical for use in the anti-skid control system in view of the cost. Therefore, a usual method for monitoring the vehicle speed in anti-skid control involves latching a wheel speed upon initiation of anti-skid control as a vehicle speed representative datum, because upon initiation of the anti-skid control the point where the wheel deceleration exceeds the wheel deceleration threshold, is approximately coincident with the vehicle speed. This vehicle speed representative datum will be hereafter referred to as "projected speed". Based on the latched value, a vehicle speed represented datum is projected utilizing a given gradient of vehicular speed which can be derived in various ways.
For example, Japanese Patent First (unexamined) Publication (Tokkai) Showa No. 60-261767 discloses one approach for deriving the gradient of vehicular speed. In the disclosed system, the gradient of vehicular speed is determined on the basis of a difference of the latched wheel speeds latched in the current and immediately preceding anti-skid brake control cycles. Such a system is generally effective in projecting the projected speed at acceptable precision level. However, a difficulty is encountered when road friction varies during the skid cycle. Namely, when the road friction changes from low friction to high friction, wheel acceleration and deceleration gradients becomes greater. In such case, since the gradient can not be adjusted during the anti-skid brake control cycle, the gradient derived with respect to low friction road continues to be used. As a result, projected speed becomes far higher than the actual speed. Therefore, anti-skid control becomes unacceptably inaccurate.